Photodetector with surface plasmon resonance

ABSTRACT

Methods and structures for providing single-color or multi-color photo-detectors leveraging plasmon resonance for performance benefits. In one example, a radiation detector includes a semiconductor absorber layer having a first electrical conductivity type and an energy bandgap responsive to radiation in a first spectral region, a semiconductor collector layer coupled to the absorber layer and having a second electrical conductivity type, and a plasmonic resonator coupled to the collector layer and having a periodic structure including a plurality of features arranged in a regularly repeating pattern.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e) to co-pendingU.S. Provisional Patent Application No. 61/605,549 titled “PHOTODETECTORWITH SURFACE PLASMON RESONANCE” filed on Mar. 1, 2012, which isincorporated herein by reference in its entirety.

BACKGROUND

Infrared detectors are used in a wide variety of applications including,for example, remote sensing, infrared astronomy, and various militaryapplications. Infrared detectors are generally sensitive to thermalnoise, and are therefore often cooled to cryogenic operatingtemperatures, for example, approximately 77 Kelvin (K). Recently, therehas been a significant interest in higher operating temperature (HOT)infrared detectors, particularly HOT infrared focal plane arrays (FPAs),to remove or reduce the need for expensive cooling systems. Currentapproaches to realizing HOT detectors have focused on either thematerial design to address fundamental mechanisms such as Augerrecombination, or reducing the volume of the detector to reducesensitivity to thermal noise. However, by focusing on only one aspect ofthe problem at a time (either recombination or volume reduction),current approaches limit their utility, and even when ideallyimplemented, these solutions do not necessarily achieve a high enoughoperating temperature to provide significant benefit.

SUMMARY OF INVENTION

Aspects and embodiments are directed to methods and apparatus forachieving a HOT detector that address both volume reduction andrecombination mechanisms in one device, thereby providing a superiorsolution. As discussed in more detail below, certain embodiments aredirected to a focal plane array or other infrared detector that includesa plasmonic resonator in combination with a reduction in thickness ofthe absorber layer of the device. These detectors may be single-color ordual-color.

According to one embodiment, a radiation detector comprises asemiconductor absorber layer having a first electrical conductivity typeand an energy bandgap responsive to radiation in a first spectralregion, a semiconductor collector layer coupled to the absorber layerand having a second electrical conductivity type, and a plasmonicresonator coupled to the collector layer and having a periodic structureincluding a plurality of features arranged in a regularly repeatingpattern.

In one example, the periodic structure of the plasmonic resonator is agrating, and the plurality of features includes a plurality of ridgeswhich may or may not be interconnected to each other. In one example,the absorber layer is an n-type semiconductor material, and thecollector layer is a p-type semiconductor material. In another example,the absorber layer is a p-type semiconductor material, and the collectorlayer is an n-type semiconductor material. In another example, the firstelectrical conductivity type of the absorber layer is one of n-type andp-type, the second electrical conductivity type of the collector layeris the same as the first electrical conductivity type, and the absorberlayer is separated from the collector layer by a barrier.

In one example, the first spectral region includes a plurality ofwavelengths including at least one first wavelength and at least onesecond wavelength that is longer than the first wavelength, wherein theabsorber layer includes a first region responsive to radiation havingthe at least one first wavelength and a second region responsive toradiation having the at least one second wavelength, and wherein theplasmonic resonator is configured to focus the radiation having the atleast one first wavelength into the first region of the absorber layer.The first region of the absorber layer may have a thicknessapproximately equal to a depletion width of the radiation detector.

In another example the absorber layer is a first absorber layer and thecollector layer is a first collector layer, and the radiation detectorfurther comprises a second semiconductor absorber layer having the firstelectrical conductivity type and a second energy bandgap responsive toradiation in a second spectral region, and a second semiconductorcollector layer coupled to the second absorber layer and positionedbetween the second absorber layer and the first absorber layer, andhaving a third electrical conductivity type. The first electricalconductivity type may be n-type, the second electrical conductivity typemay be n+-type, and the third electrical conductivity type may bep-type, for example. The first spectral region may include a firstplurality of wavelengths, and the second spectral region may include asecond plurality of wavelengths that are shorter than the firstplurality of wavelengths. In one example the second spectral regionincludes at least a portion of one of the NIR, SWIR, MWIR and LWIRspectral regions. In another example the first absorber layer has athickness approximately equal to a depletion width of the radiationdetector. In another example the radiation detector further comprises asubstrate, the absorber layer being formed on the substrate andpositioned between the substrate and the collector layer.

According to another embodiment a dual-band radiation detector comprisesa first collector layer having a first electrical conductivity type, afirst absorber layer having a second electrical conductivity type and afirst energy bandgap responsive to radiation in a first spectral regionincluding a first plurality of wavelengths, a second absorber layerhaving the second electrical conductivity type and a second energybandgap responsive to radiation in a second spectral region including asecond plurality of wavelengths longer than the first plurality ofwavelengths, the first collector layer being positioned between thefirst and second absorber layers, a third layer coupled to the secondabsorber layer and having a third electrical conductivity type, thesecond absorber layer being positioned between the third layer and thefirst collector layer, and a plasmonic resonator coupled to third layerand having a grating structure including a plurality of ridges arrangedin a regularly repeating pattern, the plasmonic resonator beingconfigured to focus the radiation in the second spectral region to thesecond absorber layer.

In one example the first collector layer comprises a p-type material,the first and second absorber layers each comprises an n-type material,and the third layer comprises an n+-type material. In another examplethe first and second spectral regions are infrared spectral regions.

According to another embodiment, a dual-band radiation detectorcomprises a first absorber layer having a first electrical conductivitytype and a first energy bandgap responsive to radiation in a firstspectral region including a first plurality of wavelengths, a secondabsorber layer having the first electrical conductivity type and asecond energy bandgap responsive to radiation in a second spectralregion including a second plurality of wavelengths longer than the firstplurality of wavelengths, a barrier layer disposed between the firstabsorber layer and the second absorber layer, and a plasmonic resonatorcoupled to second absorber layer and having a grating structureincluding a plurality of ridges arranged in a regularly repeatingpattern, the plasmonic resonator being configured to focus the radiationin the second spectral region to the second absorber layer.

Still other aspects, embodiments, and advantages of these exemplaryaspects and embodiments are discussed in detail below. Embodimentsdisclosed herein may be combined with other embodiments in any mannerconsistent with at least one of the principles disclosed herein, andreferences to “an embodiment,” “some embodiments,” “an alternateembodiment,” “various embodiments,” “one embodiment” or the like are notnecessarily mutually exclusive and are intended to indicate that aparticular feature, structure, or characteristic described may beincluded in at least one embodiment. The appearances of such termsherein are not necessarily all referring to the same embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of at least one embodiment are discussed below withreference to the accompanying figures, which are not intended to bedrawn to scale. The figures are included to provide illustration and afurther understanding of the various aspects and embodiments, and areincorporated in and constitute a part of this specification, but are notintended as a definition of the limits of the invention. In the figures,each identical or nearly identical component that is illustrated invarious figures is represented by a like numeral. For purposes ofclarity, not every component may be labeled in every figure. In thefigures:

FIG. 1 is a perspective view of one example of a portion of focal planearray of an infrared detector according to aspects of the invention;

FIG. 2 is a cross-sectional view of one example of an infrared detectoraccording to aspects of the invention;

FIG. 3A is a graph illustrating a spectral profile for the exampleinfrared detector of FIG. 2;

FIG. 3B is a diagram illustrating energy levels in the layers of theinfrared detector of FIG. 2;

FIG. 4 is a cross-sectional view of another example of an infrareddetector according to aspects of the invention;

FIG. 5A is a graph illustrating a spectral profile for the exampleinfrared detector of FIG. 4;

FIG. 5B is a diagram illustrating energy levels in the layers of theinfrared detector of FIG. 4;

FIG. 6 is a cross-sectional view of another example of an infrareddetector according to aspects of the invention;

FIG. 7A is a graph illustrating a spectral profile for the exampleinfrared detector of FIG. 6;

FIG. 7B is a diagram illustrating energy levels in some the layers ofthe infrared detector of FIG. 6;

FIG. 8 is a cross-sectional view of another example of an infrareddetector according to aspects of the invention;

FIG. 9A is a graph illustrating a spectral profile for the exampleinfrared detector of FIG. 8;

FIG. 9B is a diagram illustrating energy levels in some the layers ofthe infrared detector of FIG. 8;

FIG. 10 is a cross-sectional view of another example of an infrareddetector according to aspects of the invention;

FIG. 11A is a graph illustrating a spectral profile for the exampleinfrared detector of FIG. 10;

FIG. 11B is a diagram illustrating energy levels in some the layers ofthe infrared detector of FIG. 10;

FIG. 12 is a cross-sectional view of another example of an infrareddetector according to aspects of the invention;

FIG. 13A is a graph illustrating a spectral profile for the exampleinfrared detector of FIG. 12;

FIG. 13B is a diagram illustrating energy levels in some the layers ofthe infrared detector of FIG. 12; and

FIG. 14 is a graph illustrating simulated dark currents for the exampleinfrared detectors of FIGS. 2, 4 and 10.

DETAILED DESCRIPTION

Higher operating temp (HOT) focal plane arrays, or other infrareddetectors, may be achieved through various mechanisms, some of whichinclude reducing the volume of the detector. As discussed above,infrared detectors are sensitive to thermal noise, which is why thesedetectors are typically cooled to cryogenic operating temperatures.Noise mitigation may be achieved by volume reduction of the noisierbandgap regions within the infrared detector. However, reducing thedetector volume may result in lost performance. Accordingly, aspects andembodiments are directed to a mechanism for compensating for this lostperformance. In particular, aspects and embodiments provide an approachfor realizing a HOT detector that addresses both the relationshipbetween detector volume and quantum efficiency and the fundamentalrecombination mechanisms that limit performance at high temperatures.According to one embodiment, a HOT detector leverages surface Plasmonresonance for performance improvement. As discussed in more detailbelow, this technique may provide a powerful resonant structure to allowtwo-fold improvement as the longer wavelength absorber may be both verysmall and in some instances fully depleted.

It is to be appreciated that embodiments of the methods and apparatusesdiscussed herein are not limited in application to the details ofconstruction and the arrangement of components set forth in thefollowing description or illustrated in the accompanying drawings. Themethods and apparatuses are capable of implementation in otherembodiments and of being practiced or of being carried out in variousways. Also, the phraseology and terminology used herein is for thepurpose of description and should not be regarded as limiting. The useherein of “including,” “comprising,” “having,” “containing,”“involving,” and variations thereof is meant to encompass the itemslisted thereafter and equivalents thereof as well as additional items.References to “or” may be construed as inclusive so that any termsdescribed using “or” may indicate any of a single, more than one, andall of the described terms.

Referring to FIG. 1, there is illustrated an example of a focal planearray (FPA) 100 of infrared detectors 110. In the illustrated example,the FPA 100 includes a two-dimensional array of eight detectors 110;however those skilled in the art will appreciate, given the benefit ofthis disclosure, that the FPA may include any number of detectorsarranged in one, two or three dimensions. Additionally, each infrareddetector 110 may have any shape and dimension suitable for radiationdetection. In this example, each infrared detector 110 includes multiplesemiconductor layers 120, 130 and 140; however, as discussed furtherbelow, in other embodiments, the detectors may include more or fewersemiconductor layers. One or more substrates 150 may provide a base uponwhich the semiconductor layer(s) 120, 130 and/or 140 may be formed. Theinfrared detectors 110 may be at least partially separated from oneanother by gaps 160 in which little or no absorption occurs. Eachdetector 110 may correspond to a pixel of the FPA 100.

The substrate 150 may be a wafer comprised of silicon (Si), germanium(Ge), cadmium telluride (CdTe), cadmium zinc telluride (CdZnTe), galliumarsenside (GaAs), and/or any other suitable substrate material orcombination of substrate materials upon which the semiconductor layers120, 130, and/or 140 may be formed. The semiconductor layer(s) 120, 130and/or 140 may be formed using any suitable semiconductor process,including epitaxy, for example, such as molecular beam epitaxy,metalorganic vapor phase epitaxy or liquid phase epitaxy. At least oneof the semiconductor layers 120, 130 and/or 140 may include a materialhaving energy bandgaps responsive to radiation in a spectral region (orwaveband) of interest (referred to as an absorber layer). Some examplesof materials include, but are not limited to, Silicon, GaAs, InGaAs,HgCdTe, Lead chalcogenides, and super lattices.

According to one embodiment, one or more of the detectors 110 areconfigured to leverage surface Plasmon resonance to thin at least one ofthe semiconductor layers acting as the absorber for at least onewaveband of the detector. Referring to FIG. 2, there is illustrated anexample of a single-waveband (also referred to as single-color)photodetector 200, according to one embodiment. The photodetector 200includes a semiconductor absorber layer 210, which may be formed on asubstrate 150 using any suitable semiconductor manufacturing process, asdiscussed above, and has an energy bandgap responsive to radiation in aspectral region of interest. A semiconductor collector layer 220 iscoupled to the absorber layer 210 and provides an electrical connectionfor the photodetector 200. In the illustrated example, the absorberlayer 210 is an n-type layer and the collector layer 220 is a p-typelayer, thereby creating a p-n junction. However, it is to be appreciatedthat the electrical conductivity type of the layers 210, 220 may bereversed in other examples. In addition, as discussed further below, thedevice may be formed with an NBN-type configuration, rather than a p-n(or n-p) junction. An NBN configuration is a barrier type device where nis the doping type. The device may alternatively be formed with a PBPconfiguration, namely, a barrier type device where p is the doping type.A Plasmon resonator 230 is fabricated on the detector 200 and providesan electrical contact structure for the detector. The Plasmon resonator230 is a metal layer.

The plasmonic resonator 230 operates by resonating incident flux,causing a field to be built up in the absorptive region (absorber layer210) of the detector 200. Generated carriers are separated and collectedin the absorptive region in accord with normal operation of aphotovoltaic device. In the illustrated example, the plasmonic resonator230 is formed with a grating structure that includes protrusions orridges 235 that are periodically spaced. The dimensions of the ridges235 and period of the grating may be tailored to focus plasma waves intothe absorber layer 210, and to achieve a desired wavelength selectivityor polarization selectivity, as discussed further below. In addition,the design of the plasmonic resonator may be varied on a per pixelbasis, to provide individualized spectral and/or polarization responsesfor the pixels of a detector array. Responsive to incident radiation inthe z-direction (generally normal to the surface of the detector 200),the plasmonic resonator 230 causes a resonance in the x-y plane, therebyallowing a very thin absorber layer 210 to collect substantially allphotons and maintain a high quantum efficiency. As a result, volumereduction (thinning) of the absorber layer 210 may not hinder opticalperformance of the detector 200, thereby improving signal to noise.

In one embodiment, the resonance of the plasmonic resonator 230 allowsthe absorber layer 210 to be sufficiently thinned such that it may befully depleted or close or fully depleted at standard operatingvoltages. Depletion of the absorber layer 210 means that limitingmechanisms at high temperatures, such as Auger recombination, aresuppressed. Furthermore, as discussed above, volume reduction alsoreduces the sensitivity of the device to thermal noise, and reduces“dark current.” Dark current is the constant response exhibited by areceptor of radiation during periods when it is not actively beingexposed to light. In particular, in the context of a photodetector orphotovoltaic device, dark current refers to the relatively smallelectric current that flows through the photosensitive device when nophotons are entering the device.

As discussed above, the detector 200 illustrated in FIG. 2 is asingle-color (or single-waveband) device. There are several single-colorimplementations that may leverage surface Plasmon resonance for variousdifferent sensing applications. For example, the detector 200 of FIG. 2may provide a narrow-band sensor that may be used for resonantdetection. FIG. 3A illustrates an example spectral profile of thedetector 200 of FIG. 2 configured as an infrared detector (i.e., theabsorber layer 210 is selected to include one or more materialsresponsive to infrared radiation). FIG. 3A represents a generic spectralresponse. The specific spectral response of an exemplary device may bedependent on the combination of the absorber material cut-off wavelengthand optical properties, absorber thickness (which may be much thinnerthan a conventional absorbing layer, as discussed above), and dimensionsof the resonator. The dimensions of the resonator are typicallydetermined by the operating wavelength, material properties, and desiredresponse/sensitivity. For example, a device having a spectral responseof the form illustrated in FIG. 3A may include an MWIR absorber, acut-off wavelength of approximately 5 μm, an absorber thickness ofapproximately 500 nm, and a resonator period of approximately 0.5-2 μm.

FIG. 3B illustrates a corresponding diagram showing energy levels acrossan example of the detector 200 of FIG. 2. As discussed above, the widthW1 of the absorber layer 210 may be made very narrow, in one example,approximately the same as the depletion width of the detector, such thatthe device may be operated fully depleted. Such a sensor may realize aHOT detector with good performance through the reduction of thermalnoise, dark current, and other limiting factors, as discussed above.

According to another embodiment, a single-color photodetector may beimplemented for broad-band sensing. For example, a broad-band detectormay be implemented, leveraging the reduced dark current of the device,by confining the shortest wavelengths to a very narrow, optionally fullydepleted absorber, while allowing other wavelengths to be absorbedthrough more standard absorbers. An example of such a detector isillustrated in FIG. 4.

Referring to FIG. 4, in one example, a single-color broad-bandphotodetector 400 includes an absorber layer 410 that is divided intotwo regions, namely, region 410 a and region 410 b, as shown by thedotted line in FIG. 4. Absorber region 410 a may be responsive to acertain group of wavelengths, for example, the shorter wavelengths of awaveband of interest, and the absorber region 410 b may be sensitive toother wavelengths in the waveband of interest. The Plasmon resonator 230may be configured to focus radiation with selected wavelengths intoabsorber region 410 a. Accordingly, absorber region 410 a may be madevery thin, for example (referring to FIG. 5B), the width W2 of absorberregion 410 a may be approximately the same as the depletion width. Thus,in one example, the detector 400 may be operated with absorber region410 a fully depleted and leveraging the Plasmon resonance to achievehigh quantum efficiency.

In one example, absorber region 410 a resembles the detector 200 of FIG.2 in operation, and may be a very narrow-band detector. Absorber region410 b may have a wider spectral range (or bandwidth). For exampleabsorber region 410 a may have a peak response at 4.5 μm with an FWHMresponse of 0.5-1 μm, while absorber 410 b has a broad-band responseover the region 1-4.25 μm region. Thus, the combination of the two colorabsorbers may cover the entire useful MWIR region, while having the darkcurrent performance of only the shorter wavelength material 410 b, whichdominates dark current in the longer wavelength absorber 410 a. Absorberregion 410 b may not receive resonant energy from the Plasmon resonator230, and may absorb photons according to conventional photovoltaicprocesses. Thus, the surface Plasmon resonator 230 may be used to managewhere absorption of photons with selected wavelengths occurs within adetector device to improve performance of the device. Accordingly, abroad-band device may be achieved by using the thin, narrow-bandabsorber region 410 a for some wavelengths, and the thicker,broader-band absorber region 410 b to capture the other wavelengths. Inone example, the thickness of absorber region 410 a may be approximately300 nanometers (nm) and the thickness of absorber region 410 b may beapproximately 5 micrometers (μm).

Similar to FIG. 3A discussed above, FIG. 5A illustrates an examplegeneric spectral profile for an infrared embodiment of detector 400. Inone example, the detector 400 is configured for the infrared spectralregion extending from approximately 3 μm to 5 μm. In another example,the detector 400 is configured for the infrared spectral regionextending from approximately 8 μm to 12 μm. As discussed above, absorberregion 410 a is configured to detect a first subset of the spectralregion, similar to the detector of FIG. 2, and absorber region 410 b isconfigured to detect the remainder of the spectral region; therebyachieving a broad-band, single-color device. As discussed above, FIG. 5Billustrates the corresponding energy level diagram for the exampledetector of FIG. 4.

As discussed above, other embodiments of detectors may include NBNdetectors that instead of a p-n junction include a barrier layer betweenthe absorber and the collector. The plasmonic resonator may be formed onthe collector and the absorber may be thinned, as discussed above.

One example of an NBN single-color detector is illustrated in FIG. 6. Inthis example, the detector 600 includes an absorber layer 610, a barrierlayer 620 and a collector layer 630. The absorber layer 610 and thecollector layer 630 may have the same electrical conductivity type, forexample, n-type, and are separated from one another by the barrier layer620. As discussed above, by using the Plasmon resonator 230 to focusplasma waves into the absorber layer 610, the absorber layer may be madevery thin. In this context, “very thin” may be defined by opticalabsorption depth, rather than a physical property of the absorber layer.For example, a typical MWIR absorption depth is approximately 1-3 μm(and is wavelength dependent); therefore, a conventional absorber layermay typically be 5-10 μm thick. In contrast, according to certainaspects of the invention, the absorber layer 610 may be “very thin” inthat the absorber thickness may be much less than the absorption depth.For example, an absorber layer of 300 nm thickness is approximately 5-10times thinner than the typical MWIR absorption depth, and therefore maybe considered very thin. The absorption depth is dependent on thematerial properties, and therefore the physical thickness of a “verythin” layer may be material dependent also.

This example detector 600 may be a narrow-band device, and may have ageneric spectral response (an example of which is illustrated in FIG.7A) similar to that of the detector 200 of FIG. 2. FIG. 7B illustratesan exemplary corresponding energy level diagram for the example detector600 of FIG. 6.

A broad-band single color detector, such as that discussed above withreference to FIG. 4, may also be implemented using an NBN configuration.An example of single-color, dual-absorber detector 800 is illustrated inFIG. 8. In this example, the detector 800 includes an absorber layer810, which as discussed above, may be divided into two absorber regions810 a and 810 b, one of which (810 b) may be thinned due to the benefitsprovided by the plasmonic resonator 230. The dual-region absorber layer810 may provide a broad-band single color spectral response (an exampleof which is illustrated in FIG. 9A). In one example, the detector 800may be constructed such that the spectral response is similar to that ofdetector 400. The absorber layer 810 is separated from a collector layer830 by a barrier layer 820. As discussed above with respect to FIG. 6,in this configuration, the absorber layer 810 and collector layer 830may have the same electrical conductivity type. The plasmonic resonator230 is formed on the collector layer 830.

FIG. 9A illustrates an example of the generic spectral response ofdetector 800, and FIG. 9B illustrates a corresponding exemplary energylevel diagram. In one example, the detector 800 is configured to coverwavelength ranges from approximately 4.25-5 μm and less than 4.25 μm. Inanother example, in which InAlSb and InAsSb materials are used, thedetector 800 may be configured to cover wavelength ranges fromapproximately 3.25-4 μm and less than 4 μm. As discussed above, superlattices may also be used for the detector materials.

According to another embodiment, a two-color (or dual-band) device mayalso be implemented using a surface Plasmon resonator, as discussedabove. In one example, a dual-band detector leverages surface Plasmonresonance to thin one band of the detector, particularly the band mostsensitive to dark current and limiting higher temperature operation. Asa result, a HOT two-color or dual-band detector may be realized. In oneexample, for an infrared two-color detector, the detector may includetwo absorbing regions of different cut-off. The longer-wavelengthabsorbing region may be coupled to a Plasmon resonator, as discussedfurther below, and may be made very thin, in one example, on the orderof the depletion width of the detector. This reduces the volume of thedevice and the dark current generating sources, while maintaining highquantum efficiency, as discussed above. The shorter-wavelength absorbingregion may be a standard thickness absorber, and may not receiveresonant energy from the Plasmon resonator.

Referring to FIG. 10, there is illustrated one example of a two-colordetector 1000 including a Plasmon resonator 230. The detector 1000includes a first absorber layer 1010 comprising a material having anenergy bandgap responsive to radiation in a first spectral region, and afirst collector layer 1020, which together provide detection for thefirst spectral region (referred to as the first color detector). Thedetector 1000 further includes a second absorber layer 1030 comprising amaterial having an energy bandgap responsive to radiation in a secondspectral region. In the illustrated example, the collector layer for thesecond absorber 1030 is provided by a highly doped N+ layer 1040;however, in other examples, layer 1040 may be a p-type layer. Layers1020, 1030 and 1040 together provide the second color detector. Avariety of other suitable electrical conductivity variations may be usedfor the semiconductor layers 1010, 1020, 1030 and 1040. For example, asdiscussed further below, a dual-band detector may be implemented usingan NBN configuration, as illustrated for example in FIG. 12. The contactstructure of the second color detector is patterned to provide thesurface plasmonic resonator 230, as discussed above.

In one example, the detector 1000 is an infrared detector, and the firstcolor detector is the shorter wavelength detector and the second colordetector is the longer wavelength detector. In a particular embodiment,absorber layer 1010 may have an energy bandgap responsive to a spectralrange of approximately 0.5 μm to 5 μm, and semiconductor layer 1030 mayhave an energy bandgap responsive to a different spectral region, suchas, for example, long-wavelength infrared (LWIR). In another example,the dual-band detector 1000 may include one band covering the infraredspectral region from approximately 3 μm to 5 μm, and another bandcovering the infrared spectral region from approximately 8 μm to 12 μm.In other embodiments, semiconductor layers 1010 and 1030 may beresponsive to respective ones or more of near-infrared (NIR),short-wavelength infrared (SWIR), mid-wavelength infrared, LWIR,very-long wave infrared (VLWIR), and/or one or more other spectralregions that may or may not be within the infrared spectrum. As usedherein, NIR radiation includes a spectral region extending fromapproximately 0.5 to 1 μm, SWIR radiation includes a spectral regionextending from approximately 1 to 3 μm, MWIR radiation includes aspectral region extending from approximately 3 to 8 μm, LWIR radiationincludes a spectral region extending from approximately 8 to 12 μm, andVLWIR radiation includes a spectral region extending from approximately12 to 30 μm. Longer wavelength infrared radiation is generally moresensitive to thermal noise than is shorter wavelength infraredradiation. Accordingly, it may be advantageous to apply the benefits ofthe plasmonic resonator to the longer wavelength (second color) absorberlayer 1030. However, in other examples, particularly if the detector1000 is configured for a spectral region other than the infrared region,the second color detector may be the shorter wavelength detector.

In one embodiment, the second absorber layer 1030 is thinned, forexample, until it is approximately a depletion region thickness. Asillustrated in FIG. 10, the second absorber layer 1030 is sandwichedbetween two regions of higher band gap (and also higher doping density),namely the collector layers 1020 and 1040. In one example, the secondcolor detector is operated fully depleted. This reduces Augerrecombination, in some instances leaving only G-R recombination, whichmay be controlled through careful selection of the material quality (forthe material of absorber layer 1030) and is not a fundamental materiallimit for higher temperature operation. In one example, the second colordetector using the Plasmon resonator 230 is narrow-band, and may beconfigured for the wavelengths most sensitive to thermal noise, darkcurrent or other limiting effects. The shorter wavelength absorber layer1010 may be broad-band (as discussed above, this absorber may not beaffected by the Plasmon resonator 230) and may be used to cover thewavelengths of the absorption spectrum of interest that are not detectedby the narrow-band absorber 1030.

Another example of a dual-band radiation detector that may be modifiedto include a plasmonic resonator 230 coupled to the absorber layerassociated with one spectral band of the detector is described in U.S.Patent Publication No. 2011/0147877 titled “MULTI-BAND, REDUCED-VOLUMERADIATION DETECTORS AND METHODS OF FORMATION,” published on Jun. 23,2011 and incorporated herein by reference in its entirety.

FIG. 11A illustrates one example of a spectral profile corresponding toan infrared example of the detector 1000 of FIG. 10. In this example,the first color detector (using absorber layer 610) detects the first(shorter wavelength and broader band, for example the 3-5 μm MWIRwindow) spectral region 1110, and the second color detector (using thePlasmon resonator and absorber layer 1030) covers the second(narrow-band, longer wavelength, for example sections of the LWIR 8-12um window, with the spectral content defined by the geometry of theresonator) spectral region 1120. FIG. 11B illustrates a correspondingportion of an energy level diagram including semiconductor layers 1020,1030 and 1040, and the Plasmon resonator contact 235. As discussedabove, in one example, the width W3 of the second absorber layer 1030may be approximately the depletion thickness of the detector 1000.Examples of thickness include approximately 5-10 μm for Band 1 (thestandard absorber thickness) and 300 nm for Band 2 (with plasmonicenhancement).

As discussed above, a two-color detector may be implemented using an NBNconfiguration, as illustrated for example in FIG. 12. In this example,the detector 1200 includes a first absorber layer 1210 comprising amaterial having an energy bandgap responsive to radiation in a firstspectral region, and a second absorber layer 1230 comprising a materialhaving an energy bandgap responsive to radiation in a second spectralregion. The two absorber layers are separated from one another by abarrier layer 1220. The plasmonic resonator 230 is coupled to the secondabsorber layer 1230 induces a resonance therein, as discussed above, toallow this layer to be thinned while maintaining high quantumefficiency. FIG. 13A illustrates an example of the spectral response ofdetector 1200, which may be similar to that of detector 1000. FIG. 13Billustrates a corresponding energy level diagram for an example of thedetector 1200.

FIG. 14 is a graph showing simulated dark currents for various examplesof detectors using plasmonic resonators in accord with certainembodiments. Dark current in amperes per square centimeter (y-axis) areplotted as a function of the normalized inverse operating temperature ofthe detector (x-axis; operating temperature decreasing to the right).Trace 1410 represents the dark current for a baseline single colordetector without a Plasmon resonator (standard thickness absorber).Trace 1420 illustrates the dark current for an example of a single colorbroad-band detector, such as that illustrated in FIG. 4. Trace 1430illustrates the dark current for an example of a single colornarrow-band detector, such as that illustrated in FIG. 2. As can be seenwith reference to FIG. 14, the dark current is substantially reduced forthese example detectors utilizing the Plasmon resonance. Traces 1440 and1450 corresponding to an example two-color detector, such as that shownin FIG. 10. Trace 1440 illustrates the dark current for the firstspectral region or waveband of the detector (corresponding to absorberlayer 1010), and trace 1450 illustrates the dark current for the secondspectral region or waveband, corresponding to absorber layer 1030. Inone example, by leveraging Plasmon resonance and operating the detectorsfully depleted, an infrared detector may be made to perform withapproximately 50 times less dark current at an operating temperature of200 K than a conventional (e.g., the baseline; trace 1410) infrareddetector.

Thus, aspects and embodiments provide a single- or dual-band radiationdetector, for example, an infrared detector, in combination with aplasmonic resonator. As discussed above, the plasmonic resonator allowsvolume reduction of the absorber layer of one band (or selection ofwavelengths) of the detector, for example, the narrowest bandgapmaterial, while another absorber associated with the other band/colormay allow broad-band detection at wavelengths not within the narrow-bandspectral region influenced by the plasmonic resonator. Thus, a HOTdetector may be realized by employing the plasmonic resonator to achievea thin, optionally fully depleted, absorber for one spectral region orone or more wavelengths (e.g., the spectral region most sensitive tothermal noise or where highest resolution/performance is desired), andusing a second absorber material for broader detector response (e.g.,for a broad-band single color detector leveraging multiple colorabsorbing regions) or dual-color applications.

Furthermore, according to one embodiment, the plasmonic resonator 230may be designed to allow for selectivity in one or multiple opticalregimes. For example a single narrow-band resonance can be designed, andvaried across the focal plane array 100 for multi- or hyper-spectralimaging. Thus, referring again to FIG. 1, different detectors 110 in thefocal plane array 100 may be configured with different plasmonicresonators to achieve sensitivity in different spectral regions. Forexample, the period and/or dimensions of the ridges 235 may be variedfrom detector to detector to tailor each detector 110 to a specificwaveband. In another example, various polarization sensitivities may bedesigned into the plasmonic resonators 230, again by varying thedimensions and/or grating period of the ridges 235.

As discussed above, in some embodiments, the detectors 200, 400, 600,800, 1000 and/or 1200 are infrared detectors, and accordingly thesemiconductor layers may include materials that are capable of detectinginfrared radiation in any one or more of the NIR, SWIR, MWIR, LWIRand/or VLWIR spectral bands. One example material capable of detectingradiation is mercury cadmium telluride (HgCdTe). In one embodiment, thesemiconductor layers 120, 130, 140, 210, 410, 1010 and/or 1030 at leastpartially comprise HgCdTe in the form of Hg_((1-x))Cd_(x)Te. The x valueof the HgCdTe alloy composition may be chosen, for example, so as totune the optical absorption of the corresponding semiconductor layer tothe desired infrared wavelength. In other examples, the semiconductorlayers 120, 130, 140, 210, 410, 1010 and/or 1030 may comprise additionaland/or alternative materials responsive to radiation. For example, thesemiconductor layers 120, 130, 140, 210, 410, 1010 and/or 1030 maycomprise mercury cadmium zinc telluride (HgCdZnTe) and/or group III-Vsemiconductor materials, such as, for example, GaAs, AlGaAs, InAs, InSb,GaSb, and their alloys. As another example layers 120, 130, 140, 210,410, 1010 and/or 1030 may be based on a type-II strained-layersuperlattice structure.

Having described above several aspects of at least one embodiment, it isto be appreciated various alterations, modifications, and improvementswill readily occur to those skilled in the art. Such alterations,modifications, and improvements are intended to be part of thisdisclosure and are intended to be within the scope of the invention.Accordingly, the foregoing description and drawings are by way ofexample only, and the scope of the invention should be determined fromproper construction of the appended claims, and their equivalents.

What is claimed is:
 1. A radiation detector comprising: a semiconductorabsorber layer having a first electrical conductivity type and an energybandgap responsive to radiation in a first spectral region; asemiconductor collector layer coupled to the absorber layer and having asecond electrical conductivity type; and a plasmonic resonator coupledto the collector layer and having a periodic structure including aplurality of features arranged in a regularly repeating pattern.
 2. Theradiation detector of claim 1, wherein the periodic structure of theplasmonic resonator is a grating, and the plurality of features includesa plurality of ridges.
 3. The radiation detector of claim 2, wherein theplurality of ridges are interconnected to each other.
 4. The radiationdetector of claim 2, wherein at least one of a dimension of theplurality of ridges and a period of the grating is selected to impart apredetermined wavelength selectivity or polarization selectivity to theplasmonic resonator.
 5. The radiation detector of claim 1, wherein theabsorber layer is an n-type semiconductor material, and the collectorlayer is a p-type semiconductor material.
 6. The radiation detector ofclaim 1, wherein the absorber layer is a p-type semiconductor material,and the collector layer is an n-type semiconductor material.
 7. Theradiation detector of claim 1, wherein the first electrical conductivitytype of the absorber layer is one of n-type and p-type, the secondelectrical conductivity type of the collector layer is the same as thefirst electrical conductivity type; and wherein the absorber layer isseparated from the collector layer by a barrier.
 8. The radiationdetector of claim 1, wherein the first spectral region includes aplurality of wavelengths including at least one first wavelength and atleast one second wavelength that is longer than the first wavelength;wherein the absorber layer includes a first region responsive toradiation having the at least one first wavelength and a second regionresponsive to radiation having the at least one second wavelength; andwherein the plasmonic resonator is configured to focus the radiationhaving the at least one first wavelength into the first region of theabsorber layer.
 9. The radiation detector of claim 8, wherein the firstregion of the absorber layer has a thickness approximately equal to adepletion width of the radiation detector.
 10. The radiation detector ofclaim 1, wherein the absorber layer is a first absorber layer and thecollector layer is a first collector layer, and further comprising: asecond semiconductor absorber layer having the first electricalconductivity type and a second energy bandgap responsive to radiation ina second spectral region; and a second semiconductor collector layercoupled to the second absorber layer and positioned between the secondabsorber layer and the first absorber layer, and having a thirdelectrical conductivity type.
 11. The radiation detector of claim 10,wherein the first electrical conductivity type is n-type, the secondelectrical conductivity type is n+-type, and the third electricalconductivity type is p-type.
 12. The radiation detector of claim 10,wherein the first spectral region includes a first plurality ofwavelengths, and the second spectral region includes the secondplurality of wavelengths that are shorter than the first plurality ofwavelengths.
 13. The radiation detector of claim 12, wherein the secondspectral region includes at least a portion of one of the NIR, SWIR andMWIR spectral regions.
 14. The radiation detector of claim 10, whereinthe first absorber layer has a thickness approximately equal to adepletion width of the radiation detector.
 15. The radiation detector ofclaim 1, further comprising a substrate, the absorber layer being formedon the substrate and positioned between the substrate and the collectorlayer.
 16. A dual-band radiation detector comprising: a first collectorlayer having a first electrical conductivity type; a first absorberlayer having a second electrical conductivity type and a first energybandgap responsive to radiation in a first spectral region including afirst plurality of wavelengths; a second absorber layer having thesecond electrical conductivity type and a second energy bandgapresponsive to radiation in a second spectral region including a secondplurality of wavelengths longer than the first plurality of wavelengths,the first collector layer being positioned between the first and secondabsorber layers; a third layer coupled to the second absorber layer andhaving a third electrical conductivity type, the second absorber layerbeing positioned between the third layer and the first collector layer;and a plasmonic resonator coupled to third layer and having a gratingstructure including a plurality of ridges arranged in a regularlyrepeating pattern, the plasmonic resonator being configured to focus theradiation in the second spectral region to the second absorber layer.17. The dual-band radiation detector of claim 16, wherein the firstcollector layer comprises a p-type material, the first and secondabsorber layers each comprises an n-type material, and the third layercomprises an n+-type material.
 18. The dual-band radiation detector ofclaim 16, wherein the first and second spectral regions are infraredspectral regions.
 19. A dual-band radiation detector comprising: a firstabsorber layer having a first electrical conductivity type and a firstenergy bandgap responsive to radiation in a first spectral regionincluding a first plurality of wavelengths; a second absorber layerhaving the first electrical conductivity type and a second energybandgap responsive to radiation in a second spectral region including asecond plurality of wavelengths longer than the first plurality ofwavelengths; a barrier layer disposed between the first absorber layerand the second absorber layer; and a plasmonic resonator coupled tosecond absorber layer and having a grating structure including aplurality of ridges arranged in a regularly repeating pattern, theplasmonic resonator being configured to focus the radiation in thesecond spectral region to the second absorber layer.